The present invention relates to the measurement of temperature and more particularly concerns temperature measuring methods and apparatus of improved accuracy and greater flexibility.
For precision measurement of temperature, platinum RTD (resistance temperature detector) probes are used in conjunction with a meter to provide a display of measured temperature. The probe generally comprises a probe body of a configuration designed for a particular type of temperature measurement having a platinum resistive sensor mounted therein, the body being connected via a probe shank to a handle or other support to enable positioning of the body in contact with surface of an object of which temperature is to be measured. Resistance of the platinum sensor varies with temperature according to a known relation depending upon the purity of the platinum and, accordingly, the platinum sensor resistance may be measured by the meter to provide an indication of measured temperature. Platinum is defined by International Practical Temperature Standards adopted in 1968(IPTS-68) and by German specifications (DIN43760) for nominal values. Platinum is the interpolating temperature standard of the U.S. National Bureau of Standards. Thus, this invention's thrust is to relate all measurements made by probes in any configuration to standards. The meter is calibrated according to the known variation of sensor resistance with temperature variation and will readily provide a readout in degrees based upon the measured resistance.
Different types of resistance probes are generally made for interchangeable use with a single meter. Several different probe, shank and tip configurations may be employed to measure temperature of different surfaces of objects. A different configuration of probe may be employed for measurement of a material that may be penetrated by insertion of the probe. Although there may be as many as twenty different types of probes made for use with a single meter, the probes are generally of two broad configurations, those that may be termed "surface probes" and those termed "immersion probes". In use, an immersion probe has its heat sensitive portion completely immersed within or penetrating the body or medium of which temperature is to be measured. The surface probe, on the other hand, can only have a portion of the external probe body surface in contact with the surface of the object of which temperature is to be measured. Accordingly, in the immersion probe, heat of the object to be measured can be transferred to the platinum sensor within the body of the probe tip without loss to the surrounding medium. With the surface probe, on the other hand, heat must be transferred to the platinum sensor within the probe tip through only that part of the probe body that is in contact with the surface to be measured. Other portions of the probe body will not be in contact with the surface to be measured, but will be in contact with the ambient medium, the surrounding air. Heat absorbed by the surface probe body from the surface to be measured is transferred both to the platinum sensor and to the surrounding air. To the extent that heat is transferred to the surrounding air from the probe body, the latter may be at a lower temperature than the temperature of the surface to be measured. It follows that in the surface probe there exists a temperature differential between the unknown temperature of the surface contacted by the probe body and the temperature of the platinum sensor. This temperature differential exists even long after the termination of the inherent response time of the probe, namely, the time required for the probe to substantially attain its final temperature.
Because the temperature measurement made with a resistive element measures resistance based upon the temperature of the sensor element itself rather than the temperature of the object being measured, and because the temperature of the sensor element in a surface probe is often less than the temperature of the surface being measured (where ambient temperature is below the measured temperature) the surface temperature probe body must be carefully manufactured with a view toward minimizing, to the greatest extent possible, heat lost by the probe body.
Careful, precise and costly manufacture of a surface probe tends to decrease these heat losses and to change concommitant heat measurement errors to acceptable magnitudes, in many applications. But even so, a single meter adapted for alternative use with immersion and surface probes will provide disparate readings for the two probes.
In certain surface probe configurations, such as those employing a coil of platinum wire cemented to a support in the manner generally illustrated in the patent to Werner et al U.S. Pat. No. 3,114,125, it is possible, with great care in manufacture, to hold heat losses of the surface probe to a small amount. However, when employing a probe of a different configuration involving a platinum wire coil of different construction or orientation, or when employing a platinum resistive sensor in the form of a thin film of platinum deposited in a serpentine configuration upon a substrate as in U.S. Pat. Nos. 4,129,848, 4,050,052 and 4,103,275, for example, a surface probe configuration may be subject to unacceptably high losses. These losses may be as much as one to ten percent, making such configurations unacceptable for precision temperature measurement. Speed of response may also be decreased.
Accordingly, it is an object of the present invention to provide precision temperature measurement that eliminates or significantly decreases abovementioned problems. Among other objects is the provision of a useful temperature measuring system in which a wide variety of probe sensors may be used interchangeably with the same meter, without specific recalibration each time a probe is changed. This feature is a matter of serviceability and confidence to the user. It is another object to provide surface sensor probes that accurately measure surface temperature for which they were designed. Over one half of the probes sold are for measurement of surface temperature. Measuring surface temperature requires a different treatment of design factors than immersion sensors.